Before Marvel ruled super hero movies, before a man could build a suit in a cave with a box of scraps, there was... Rocket Man. Then Elton John came out in 1972, and the movie The Rocketeer, which was basically a Captain America knockoff, released in 1991
Kids costumes like this are actually pretty simple in concept. Like most child related costumes and entertainment, it involves cardboard and some arts and crafts time. At minimum you'll need a decent sized box for the torso, some cardboard to make the wings, and a smaller box to make a helmet. You don't need to get too fancy here, because kids love playing with boxes.
That said, feel free to get as involved as you want. Add little accessories like antenna, grab another box to make legs, paint the boxes, and so on as you desire. You're helping your child reach the stars, after all, so why not have fun with it, too?
If you're a teen or adult your you need to get a cool rocket suit. Cardboard is the minimum once again, but if you want to get more involved, old piping, kitchen implements, and spare parts in the garage and tool shed can add detail and accessories as desired. An old oil funnel and some aluminum foil makes for a need helmet, for example.
Old flexible vent tubing can be used to make legs, and if you have some spare sheet metal, you may as well make some really nice wings. If you have an old visor or face shield, you're helmet just got a lot cooler, too. As with the kid, your costume can be as involved or basic as you desire, though you should paint it if you have the time. Also, if you go for the metallic look, or have a spray can of the color, for next year you can take off the wings and go out as the Tin Man from Wizard of Oz.
While in theory you could probably find a premade costume ready to go, the concept is versatile enough, and materials easy enough to find, that you should have some fun and make it yourself. Premade costumes can get pricey. With some spare cardboard and time, you can make a fun costume that will last as long as you need and don't have to feel bad about when it get's trashed.